Gas-to-Liquid Technology

ABSTRACT

A gas-to-liquids process and plant for treating natural gas, in which the natural gas is subjected to expansion through a flow restrictor so as to undergo cooling through the Joule Thomson effect, enables liquids to be separated from the gas stream. The natural gas may be cooled before it reaches the flow restrictor by heat exchange with fluid that has passed through the flow restrictor. This decreases the proportion of longer-chain hydrocarbons in the natural gas, which may simplify subsequent processing, and may enable the size of the plant to be decreased.

The present invention relates to a plant and a process for treatingnatural gas to produce a liquid product.

It is well known that most oil wells also produce natural gas. At manyoil wells natural gas is produced in relatively small quantities alongwith the oil. When the quantities of this associated gas aresufficiently large or the well is close to pre-existing gastransportation infrastructure, the gas can be transported to an off-siteprocessing facility. When oil production takes place in more remoteplaces it is difficult to introduce the associated gas into existing gastransportation infrastructure. In the absence of such infrastructure,the associated gas has typically been disposed of by flaring orre-injection. However, flaring the gas is no longer an environmentallyacceptable approach, while re-injection can have a negative impact onthe quality of the oil production from the field.

Gas-to-liquids technology can be used to convert the natural gas intoliquid hydrocarbons and may follow a two-stage approach to hydrocarbonliquid production comprising syngas generation, followed byFischer-Tropsch synthesis. In general, syngas (a mixture of hydrogen andcarbon monoxide) may be generated by one or more of partial oxidation,auto-thermal reforming, or steam methane reforming. Where steam methanereforming is used, the reaction is endothermic and so requires heat. Thesyngas is then subjected to Fischer-Tropsch synthesis. For performingFischer-Tropsch synthesis the optimum ratio of hydrogen to carbonmonoxide is about 2:1, and steam reforming has a benefit of providingmore than sufficient hydrogen for this purpose.

Such a process is described for example in WO 01/51194 (AEA Technology)and WO 03/006149 (Accentus plc). Natural gas is primarily methane, butalso contains small proportions of longer-chain hydrocarbons. In eachcase the natural gas is first subjected to a pre-reforming step in whichthe longer-chain hydrocarbons are converted to methane by reaction withsteam, for example over a nickel catalyst at 400° C. As regards theFischer-Tropsch process, as described in WO 2004/050799 (GTLMicrosystems AG), a suitable catalyst uses small particles of cobalt ona ceramic support, but this catalyst can suffer a deleterious reactionin the presence of water vapour. To ensure this does not occur thereactor is operated so as to ensure the Fischer-Tropsch conversion is nomore than 70%, and then the resulting gases are subjected to a secondFischer-Tropsch stage. Although this provides a satisfactory way ofconverting natural gas to a longer-chain hydrocarbon product, it wouldbe desirable to provide an alternative plant and process.

According to the present invention there is provided a gas-to-liquidsplant for treating natural gas, wherein the natural gas is subjected toexpansion through a flow restrictor so as to undergo cooling through theJoule Thomson effect, with separation of resulting liquids.

The present invention also provides a process for treating natural gasin this way. The process of the invention relates to a chemical processfor converting natural gas (primarily methane) to longer chainhydrocarbons.

Ideally the expansion takes place without significant transfer of heatfrom the surroundings, the natural gas expanding into a lower pressurestate. The flow restrictor may be a throttle valve, or alternatively maybe an inlet nozzle of a vortex tube separator, or a turbo expander, or aTwister™ separator device. A vortex tube, or Ranque-Hilsch tube, splitsthe gas into a hot gas stream and a cold gas stream. The hot gas streammay be utilised elsewhere within the plant. However, as a result of thedivision into two streams, only a proportion of the gases are cooled.The expansion can cool the natural gas to below 0° C., more particularlybelow −10° C. for example below −15° C., with the consequence thatlonger-chain hydrocarbons condense from the vapour state into the liquidstate, and can be separated from the remaining natural gas. The degreeof cooling is selected to ensure that the output stream is at a pressuresufficient to drive gas through the process. Whilst the temperature canbe dropped further in order to increase the recovery of higherhydrocarbons, this ceases to be advantageous when the cost of theincreased compression requirements to re-pressurize the gas exceed thevalue of the additional longer chain hydrocarbons recovered. Preferablythe natural gas stream is fed into the flow restrictor through a heatexchanger in which it is cooled by contact with at least one fluid thathas been cooled by passage through the flow restrictor, so that thenatural gas is below ambient temperature when it reaches the flowrestrictor.

A benefit of this process is that the proportion of longer-chainhydrocarbons in the remaining natural gas is considerably reduced. Itmay therefore be practicable to then subject the natural gas toreforming without the need for a separate pre-reformer. A furtherbenefit is that the quantity of hydrocarbons subjected to the subsequentchemical processes is reduced, which may reduce the size and hence thecost of the remainder of the plant.

A potential problem in such a cooling process is the risk of formationof methane-containing hydrates, although this may not be an issue with aTwister device as the residence time may be sufficiently short toprevent formation of hydrate crystals. To address this issue, oxygenatessuch as methanol or ethanol may be introduced into the natural gasstream upstream of the flow restrictor. These prevent the formation ofhydrates. In the context of a gas-to-liquid plant, such oxygenates areproduced during the Fischer-Tropsch synthesis, and can be extracted fromthe resulting aqueous phase by steam stripping. These oxygenates henceallow the gas stream to be cooled to a lower temperature. The naturalgas is then converted to synthesis gas either by steam methanereforming, partial oxidation or auto-thermal reforming. In the case ofsteam methane reforming, the requisite heat may be provided by catalyticcombustion within adjacent channels within an integratedreforming/combustion reactor, or by hot exhaust gases from a separatecombustion reactor. The resulting synthesis gas contains more hydrogenthan is required for Fischer-Tropsch synthesis, and at least some excesshydrogen may be separated from the synthesis gas by a membraneseparator, and supplied to a fuel header. If a membrane is not used forseparation, separation can be performed by pressure swing absorption.The fuel header may supply the fuel for the combustion process thatprovides the heat for the steam methane reforming reaction, or mayprovide fuel for preheating an air supply for such a combustion process.

The synthesis gas may then be subjected to a Fischer-Tropsch synthesisreaction to convert the synthesis gas into longer chain hydrocarbons.This may be a single stage process or a two-stage process. Afterseparating liquid hydrocarbon product and an aqueous phase, for examplein a tubular heat exchanger followed by separation by densitydifferences in a vessel, there is a resulting tail gas. The tail gascontains hydrogen, carbon monoxide, carbon dioxide and methane. Some ofthe tail gas is preferably fed into the synthesis gas stream, preferablyupstream of the membrane separator. At least some of the tail gas is fedinto the fuel header.

At least some of the aqueous phase may be boiled to produce a steam andoxygenate-containing vapour, which is fed into the combustion gasmixture. For example it may be fed into a stream of combustion airsupplied to combustion channels or to a combustion reactor.

It will be appreciated that the steam/methane reforming process producestwo hot out-flowing streams: a synthesis gas stream typically at above800° C., and an exhaust gases stream which may be at a similartemperature (if an integrated combustion/reforming reactor is used) orwhich may be at a somewhat lower temperature (if a separate combustionreactor is used). Preferably these hot streams are used to providethermal energy to gases being supplied to the reforming process. Forexample the hot synthesis gas may be used to provide the steam requiredfor reforming, while the exhaust gases may be used to preheat combustionair.

In a further aspect of the present invention there is provided agas-to-liquid process, and a plant to perform the process, wherein theprocess utilises a combustion step using a stream of air, and whereinthe process produces excess hydrogen, wherein excess hydrogen isseparated using a membrane or using pressure swing absorption, and isused to preheat the combustion air stream. The preheating of the airstream may utilise a catalytic combustion process. For example thehydrogen may be separated from a synthesis gas or from a tail gas of aFischer-Tropsch synthesis reaction.

In yet a further aspect the present invention provides a gas-to-liquidprocess, and a plant to perform the process, wherein the processutilises a combustion step, and where the process performsFischer-Tropsch synthesis, wherein heat from the Fischer-Tropschsynthesis is used to generate steam, and the steam is introduced into agas stream supplied to the combustion step. The steam that is suppliedto the combustion step may include oxygenates that are produced as abyproduct of the Fischer-Tropsch synthesis.

The invention will now be further and more particularly described, byway of example only, and with reference to the accompanying drawingwhich shows a schematic flow diagram of a gas-to-liquid plant andassociated equipment.

The invention relates to a chemical process for converting natural gas(primarily methane) to longer chain hydrocarbons. It is suitable fortreating associated gas, which is natural gas that is produced alongwith crude oil, and is then separated from the crude oil. The firststage of the chemical process involves the formation of synthesis gas,for example by steam reforming, by a reaction of the type:

H₂O+CH₄→CO+3 H₂   (1)

This reaction is endothermic, and may be catalysed by a rhodium orplatinum/rhodium catalyst in a first gas flow channel. The heat requiredto cause this reaction may be provided by catalytic combustion of a gassuch as methane or hydrogen, which is exothermic, in an adjacentchannel, or by heat exchange with exhaust gases from a separatecombustion reactor. The combustion may be catalysed by a palladiumcatalyst in an adjacent second gas flow channel in a compact catalyticreactor. In both cases the catalyst may be on a stabilised-aluminasupport which forms a coating typically less than 100 μm thick on ametallic substrate. Alternatively, the catalyst may be applied to thewalls of the flow channels or may be provided as pellets within the flowchannel. The heat generated by the combustion would be conducted throughthe metal sheet separating the adjacent channels.

The gas mixture produced by the steam/methane reforming is then used toperform a Fischer-Tropsch synthesis to generate a longer chainhydrocarbon, that is to say:

n CO+2n H₂→(CH₂)_(n) n H₂O   (2)

which is an exothermic reaction, occurring at an elevated temperature,typically between 190° C. and 280° C., for example230° C., and anelevated pressure typically between 1.8 MPa and 2.6 MPa (absolutevalues), for example 2.5 MPa, in the presence of a catalyst such asiron, cobalt or fused magnetite, with a potassium promoter. Whilst Febased catalysts can be used, metallic Co promoted with precious metalssuch as Pd, Pt, Ru or Re doped to 1 wt % are preferred when operating atlower temperatures as they have enhanced stability to oxidation. Theactive metals are impregnated to 10-40 wt % into refractory supportmaterials such as TiO₂, Al₂O₃ or SiO₂ which may be doped with rare earthand transition metal oxides to improve their hydrothermal stability.

1. Pre-Treatment

Referring to FIG. 1, there is shown a gas-to-liquid plant 10 of theinvention. A natural gas feed 5 consists primarily of methane, but withsmall proportions of other gaseous hydrocarbons, hydrocarbon vapours,and water vapour. The gas feed 5 may for example be at a pressure of 4.0MPa (40 atmospheres) and 35° C., following sea water cooling from aninitial temperature of 90° C., and may constitute associated gas from awell producing crude oil. The natural gas feed 5 is first passed througha coalescer 12 which removes any droplets. A small quantity ofoxygenates (primarily ethanol and methanol, and marked “alcohol”) isthen sprayed into the gas feed 5 at an injector 14, and the gas feed 5is then passed through a heat exchanger 15 to cool it, and then througha throttle valve 16 through which it expands into a lower pressureregion (typically at about 1 MPa) adiabatically, with no significantheat input from the surroundings. Consequently, in accordance with theJoule Thomson effect, the natural gas is considerably cooled, forexample to −18° C. The resulting cooled stream is then fed into a phaseseparator 18, so producing a gas phase 20, a liquid hydrocarbon phase21, and an aqueous phase 22 (containing the oxygenates). The use of theoxygenates ensures that methane-containing hydrates are not produced.All three of these fluids streams are passed through the heat exchanger15 so as to cool the in-flowing gas feed 5.

The liquid hydrocarbon phase 21 constitutes part of the product outputstream of liquid hydrocarbon from the plant 10.

The gas phase 20 is then subjected to pretreatment 25, which maycomprise one or more of the following: changing its pressure; changingits temperature; and removing impurities such as sulphur. It is thenmixed with steam in a mixer 26.

2. Making Synthesis Gas

The gas/steam mixture, preferably at a temperature of about 450° C., isthen fed into a catalytic steam/methane reformer 30. The reformer 30consists of a compact catalytic reactor formed from a stack of platesdefining two sets of channels arranged alternately. One set of channelsare for the reforming reaction, and contain a reforming catalyst onremovable corrugated metal foil supports, while the other set ofchannels are for the provision of heat.

In this example the heat is provided using a separate burner 32, theexhaust gases from the burner 32 at about 850° C. being passed throughthe reformer 30 in counter-current to the flow of the steam/methanemixture. The reaction channels of the reformer 30 may contain a nickelcatalyst in an initial part of the channel, of length between 100 and200 mm, for example 150 mm, out of a total reaction channel length of600 mm. In the first part of the channel, where the nickel catalyst ispresent, pre-reforming takes place, so any higher hydrocarbons willreact with steam to produce methane. The remainder of the length of thereaction channels contains a reformer catalyst, for example aplatinum/rhodium catalyst, where the steam and methane react to formcarbon monoxide and hydrogen.

The heat for the steam/methane reforming reaction in the reformer 30 isprovided by combustion of a fuel gas from a fuel header 34 in a streamof combustion air. In this example the fuel gas is primarily hydrogen.The combustion air is provided by a blower 36 and is preheated in a heatexchanger 38, taking heat from the hot exhaust gases from the combustionafter they have passed through the reformer 30. In addition a mixture ofsteam and alcohol vapour 40 is introduced into the combustion airupstream of the burner 32. After passing through the heat exchanger 38the exhaust gases may be vented through a stack 39.

A mixture of carbon monoxide and hydrogen at above 800° C. emerges fromthe reformer 30, and is quenched to below 400° C. by passing it througha steam-raising heat exchanger 42 in the form of a thermosiphon. Theheat exchanger 42 is a tube and shell heat exchanger, the hot gasespassing through the tubes, and with inlet and outlet ducts communicatingwith the shell at the top and bottom, and communicating with a steamdrum 44. The steam drum 44 is about half full of water, and so watercirculates through natural convection between the heat exchanger 42 andthe steam drum 44. The resulting steam from the steam drum 44 issupplied to the mixer 26 through a control valve 46.

The gas mixture, which is a form of synthesis gas, may be subjected tofurther cooling (not shown). It is then subjected to compression usingtwo successive compressors 50, preferably with cooling andliquid-separation stages (not shown) after each compressor 50. Thecompressors 50 raise the pressure to about 2.5 MPa (25 atm).

It will be appreciated from equation (1) above that the ratio ofhydrogen to CO produced in this way is about 3:1, whereas thestoichiometric requirement is about 2:1, as is evident from equation(2). The high-pressure synthesis gas is therefore passed by ahydrogen-permeable membrane 52 to remove excess hydrogen. This hydrogenis supplied to the fuel header 34, and is the principal fuel gas.

3. Fischer-Tropsch Synthesis And Product Treatment

The stream of high pressure carbon monoxide and hydrogen is then heatedto about 200° C. in a heat exchanger 54, and then fed to a catalyticFischer-Tropsch reactor 55, this again being a compact catalytic reactorformed from a stack of plates as described above; the reactant mixtureflows through one set of channels, while a coolant flows through theother set. The coolant is circulated by a pump 56 and through a heatexchanger 58. The Fischer-Tropsch reaction takes place at about 210° C.,and the coolant is circulated at such a rate that the temperature variesby less than 10 K on passage through the reactor 55.

The reaction products from the Fischer-Tropsch synthesis, predominantlywater and hydrocarbons such as paraffins, are cooled to about 70° C. tocondense the liquids by passage through a heat exchanger 60 and fed to aseparating chamber 62 in which the three phases water, hydrocarbons andtail gases separate. The aqueous phase contains water with about 1-2%oxygenates such as ethanol and methanol which are formed by theFischer-Tropsch synthesis. Most of the aqueous phase from the separatingchamber 62 is treated by steam stripping 63 to separate the oxygenates(marked “alcohol”) to leave clean water that may be discharged to waste.The separated oxygenates, which are at an oxygenate concentration ofabout 80%, are injected into the injector 14 upstream of the throttlevalve 16 as described above. The remainder of the aqueous phase is fedas process water through the heat exchanger 58, and hence through apressure-drop valve 64 into a stripper tank 66. In the stripper tank 66the aqueous phase boils, typically at a pressure of about 1.0 MPa (10atm), the liquid phase being fed from the bottom of the stripper tank 66into the steam drum 44, while the vapour phase, which contains steam andthe bulk of the oxygenates, provides the stream 40 that is introducedinto the combustion air through a control valve 68.

The hydrocarbon phase from the separating chamber 62 is the longer-chainhydrocarbon product. The vapour and gas phase from the separatingchamber 62 is fed through two successive cooling heat exchangers 70, thesecond of which cools the vapours to ambient temperature. Any liquidsthat condense on passage through the first heat exchanger 70 are fedback into the separating chamber 62. The output from the second heatexchanger 70 is fed into a phase separating chamber 72, where the waterand light hydrocarbon product liquid separate.

The remaining vapour phase, which is at the same pressure as theFischer-Tropsch reactor 55, is then passed through a heat exchanger 74to a throttle valve 76 followed by a phase separating vessel 78. As thegas passes through the throttle valve 76 it expands into a lowerpressure region adiabatically, with no significant heat input from thesurroundings. Consequently, in accordance with the Joule Thomson effect,the gas is cooled considerably. The liquids that emerge from the phaseseparating pressure 78 contain water and light hydrocarbon product. Thegases that emerge from the phase separating vessel 78, which are thetail gases from the Fischer-Tropsch process, are passed back through theheat exchanger 74 to cool the in-flowing gases and, optionally, througha hydrogen permeable membrane (not shown). Part of the tail gas may befed back into the synthesis gas stream upstream of the first compressor50. At least part of the tail gas is fed into the fuel header 34, toensure that there is no excessive build-up of methane in theFischer-Tropsch reactor 55.

4. Energy Transfer And Process Review

The fuel header 34 not only provides the fuel for the burner 32, butalso supplies fuel via a fuel compressor 80 to a gas turbine 82. Indeedcompressed fuel gas may also be supplied to other equipment (not shown)that does not form part of the plant 10. The gas turbine 82 may bearranged to provide electrical power for operating the plant 10. Asindicated by a broken line in the figure, in this example the electricalpower generated by the gas turbine 82 is used to power the compressors50. Alternatively the gas turbine 82 may be coupled directly to drivethe compressors 50.

It will be appreciated that in the above-described process the heatproduced by the Fischer-Tropsch reaction is used in boiling steam andalcohol in the stripper tank 66, and so is transferred to the combustionchannels by the feed 40. The remaining heat required for reforming isprovided by the fuel gas from the fuel header 34 undergoing combustionin the burner 32. This may be a duct burner, in which there are severalnozzles through which fuel gas is fed into a stream of combustion air,so it burns with a flame. Alternatively the burner 32 may be a catalyticflame-less combustion unit. As previously mentioned, the resulting hotexhaust gas supplies heat to the reformer 30, and is then used topreheat combustion air in the heat exchanger 38.

In a modification to the above-described process the combustion air maybe additionally pre-heated by introducing hydrogen into the combustionair, and passing it through a honeycomb structure of analuminium-containing ferritic steel such as Fecralloy, with an oxidisedsurface, which has been found to be a catalytic to hydrogen combustion.Alternatively the preheating may be performed using a duct burner inwhich a fuel such as hydrogen is burnt. The final heating of thecombustion air to the desired temperature in the vicinity of 800° C. maythen be achieved using a duct burner 32 to which a combustible gasstream is provided, such as the tail gas from the Fischer-Tropschprocess, or as described above using a burner 32 which is a catalyticflame-less combustion unit.

In a further modification, the combustion air is preheated either bypassage through a duct burner provided with a suitable fuel, or byintroducing hydrogen into the combustion air, and passing it through ahoneycomb structure of aluminium-containing ferritic steel as describedabove, so that hydrogen undergoes catalytic combustion. The hotcombustion air is then fed into an integrated combustion/reformingreactor, and a fuel such as methane or tail gas is introduced andsubjected to catalytic combustion in the heat-providing channels of thereactor through which the hot combustion air flows. In this modificationthe catalytic combustion may, for example, take place over apalladium/platinum catalyst within the heat-providing channels withinthe reforming reactor 30. In this case the combustion gas path ispreferably co-current relative to the reformer gas path. The catalystmay include gamma-alumina as a support, coated with a palladium/platinum3:1 mixture, which is an effective catalyst over a wide temperaturerange. The combustible gas mixture may be supplied in stages along thereactor 30 to ensure combustion occurs throughout the length of thecombustion channels.

1-19. (canceled)
 20. A gas-to-liquids plant for treating natural gas,wherein the natural gas is subjected to expansion through a flowrestrictor so as to undergo cooling through the Joule Thomson effect,with separation of resulting longer-chain hydrocarbon liquid fromremaining natural gas, wherein the plant further comprises means toprocess the remaining natural gas to form a synthesis gas, and aFischer-Tropsch reactor to which the synthesis gas is provided, andmeans to separate hydrocarbon product and aqueous phase from the outputof the Fischer-Tropsch reactor, and means for separating oxygenates fromthe aqueous phase and means to introduce the oxygenates into the naturalgas stream upstream of the flow restrictor.
 21. A plant as claimed inclaim 20 comprising a heat exchanger arranged for heat transfer betweenthe natural gas before it reaches the flow restrictor, and at least onefluid that has been cooled by passage through the flow restrictor.
 22. Aplant as claimed in claim 20 wherein the synthesis gas formation meansforms a synthesis gas containing excess hydrogen, and wherein the plantfurther comprises a separator to remove hydrogen from the synthesis gas.23. A plant as claimed in claim 22 including a fuel header to whichseparated hydrogen is supplied.
 24. A plant as claimed in claim 23wherein the synthesis gas formation means is adapted to use anendothermic reaction, wherein heat for the reaction is provided at leastin part by combustion of fuel from the fuel header.
 25. A plant asclaimed in claim 20 comprising means to separate tail gas from theoutput from the Fischer-Tropsch reactor, wherein some of the tail gas isrecirculated into the synthesis gas stream, and some of the tail gas isfed into the fuel header.
 26. A plant as claimed in claim 20 alsocomprising a heat exchanger whereby at least some of the heat producedby the Fischer-Tropsch reaction is used to generate steam, and whereinthe steam is combined with an air stream for a combustion process.
 27. Aplant as claimed in claim 20 comprising a steam stripper to separate theoxygenates from the aqueous phase.
 28. A gas-to-liquids process fortreating natural gas, including the steps of subjecting the natural gasto expansion through a flow restrictor so as to undergo cooling throughthe Joule Thomson effect, followed by separating the resultinglonger-chain hydrocarbon liquid from the remaining natural gas, furthercomprising the steps of processing the natural gas to form a synthesisgas, and subjecting the synthesis gas to Fischer-Tropsch synthesis, andwherein the output from the Fischer-Tropsch synthesis is separated intoa hydrocarbon product and an aqueous phase, wherein the aqueous phase istreated to extract the oxygenates which are then injected into thenatural gas stream upstream of the flow restrictor.
 29. A process asclaimed in claim 28 including the step of transferring heat between thenatural gas before it reaches the flow restrictor, and at least onefluid that has been cooled by passage through the flow restrictor.
 30. Aprocess as claimed in claim 28 wherein the step of processing thenatural gas to form a synthesis gas results in a synthesis gascontaining excess hydrogen, and further comprising the step of removingexcess hydrogen from the synthesis gas.
 31. A process as claimed inclaim 30 wherein the synthesis gas formation process uses an endothermicreaction, wherein the process comprises the step of providing heat forthe endothermic reaction at least in part by combustion of the removedexcess hydrogen.
 32. A process as claimed in claim 28, wherein a tailgas is separated from the output from the Fischer-Tropsch synthesis, andwherein some of the tail gas is recirculated into the synthesis gasstream, and some of the tail gas is used as a fuel.
 33. A process asclaimed in claim 28 wherein at least some of the heat produced by theFischer-Tropsch reaction is used to generate steam, and wherein thesteam is combined with an air stream for a combustion process.
 34. Aprocess as claimed in claim 31 wherein an air stream for combustion ispreheated by mixing with hydrogen from the removed excess hydrogen, andpassing through a catalytic structure comprising an aluminium-containingferritic steel without any catalytic coating, so that the hydrogenundergoes catalytic combustion at the surface of the steel.
 35. Aprocess as claimed in claim 28 wherein the extraction treatmentcomprises steam stripping.
 36. A gas-to-liquids plant for treatingnatural gas, wherein the natural gas is subjected to a single stageexpansion through a flow restrictor so as to undergo cooling through theJoule Thomson effect, with separation of resulting longer-chainhydrocarbon liquid from remaining natural gas.
 37. A gas-to-liquidprocess for treating natural gas, including the step of subjecting thenatural gas to a single expansion through a flow restrictor so as toundergo cooling through the Joule Thomson effect, followed by separatingthe resulting longer-chain hydrocarbon liquid from the remaining naturalgas.